The present invention relates to brake assemblies, and more particularly, to brake assemblies of the type intended for use with fluid pressure actuated devices such as hydrostatic motors. Although the present invention is not necessarily limited to being used with a fluid pressure actuated motor, the invention does rely, in part, on the presence of pressurized fluid for its operation, and therefore, the invention will be described in connection with a hydrostatic motor.
Although the present invention may be included advantageously with many different types of fluid pressure actuated devices, it is especially adapted for use with a low-speed, high-torque (“LSHT”) gerotor motor, and will be described in connection therewith. As is well known to those skilled in the art, brake assemblies have become an important feature of many LSHT gerotor motors, especially when such motors are utilized for vehicle propel applications. Many vehicles propelled by hydrostatic drive circuits, which include LSHT gerotor motors, are operated on hilly terrain and on work sites involving grades, such that some sort of motor braking capability is extremely desirable, if not essential, for the safe operation of the vehicle.
In many vehicle applications for LSHT gerotor motors, the motor can have a device referred to as either a parking brake or a parking lock, the term “lock” being preferred in some instances, because it is intended that the device be engaged only after the vehicle is stopped. In other words, such parking “lock” devices are not intended to be dynamic brakes, which would be engaged while the vehicle is still moving, to bring the vehicle to a stop. However, the term “brake” has also been used, and will generally be used hereinafter to mean and include both brakes and locks. The term “brake” is somewhat preferred, to refer to a device which can be applied gradually, and to distinguish from a device which would operate only fully engaged or fully disengaged.
Examples of LSHT gerotor motors incorporating brake arrangements are illustrated and described in U.S. Pat. Nos. 6,062,835; 6,132,194; and 6,321,882, all of which are assigned to the assignee of the present invention and incorporated herein by reference. The brake arrangements in the above-incorporated patents are all of the “spring-applied, pressure-released” type in which a spring biases the brake arrangement into its “engaged” condition, braking the motor output. Although the present invention is not strictly limited to use with a brake arrangement of the spring-applied, pressure-released type, such is the most common arrangement, and the invention will be illustrated and described in connection therewith.
In order to move the brake arrangement to its “disengaged” condition, permitting normal output shaft rotation, fluid pressure must be applied to a piston seated against the biasing spring, the fluid pressure biasing the piston to overcome the force of the biasing spring, moving the piston and spring to a retracted position. As is now well known to those skilled in the art, it is preferable to make the release piston area as large as possible, thereby reducing the required release pressure. However, as the release piston is made larger, the entire brake assembly becomes larger, more complicated and more expensive.
Many gerotor motors are utilized in “closed loop” hydrostatic systems in which there is some sort of charge pump providing a pilot pressure which may serve as the release pressure for the brake arrangement. However, many other LSHT gerotor motors are, instead, utilized in “open loop” hydrostatic systems in which there is no charge pump or other source of such a pilot pressure. For motors which are to be utilized in open loop systems, it is desirable to be able to utilize system pressure (i.e., the high pressure being communicated from the pump to the work circuit) as the release pressure for the brake assembly. However, in such an arrangement, the portion of system pressure required to move the release piston to its disengaged position represents a “loss” of pressure available to be converted into motor output torque. Therefore, it is desirable to have as large a piston as possible, and as low a release pressure as possible.
Unfortunately, if the release piston is made relatively large, in order to be able to disengage the brake at a very low release pressure, there is a serious potential problem when system pressure increases to 3000 psi or 4000 psi or perhaps even more. Full, relatively high system pressure acting on a large brake release piston area would result in many thousands of pounds of axial separating force within the brake assembly housing, far beyond what the brake assembly housing would be able to withstand, in the absence of extreme and very expensive measures to strengthen the brake assembly housing, and related components.
One known solution is to place a pressure reducing or relieving valve between the source of system pressure and the release chamber of the brake assembly, to make sure that the pressure in the release chamber would never exceed some predetermined, maximum pressure. However, such a pressure reducing or relieving valve would add substantially to the overall cost and complexity of the motor and brake assembly and of its plumbing installation.